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Fair Value Hierarchy for Financial Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | $ 175,706 | $ 187,813
Money Market Funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 22,697 | 22,543
U.S. Treasuries |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 37,176 | 45,534
U.S. Government Agency Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 45,259 | 86,820
Corporate Debt Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 70,574 | 32,916
Fair Value, Inputs, Level 1 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 22,697 | 22,543
Fair Value, Inputs, Level 1 | Money Market Funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 22,697 | 22,543
Fair Value, Inputs, Level 2 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 153,009 | 165,270
Fair Value, Inputs, Level 2 | U.S. Treasuries |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 37,176 | 45,534
Fair Value, Inputs, Level 2 | U.S. Government Agency Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | 45,259 | 86,820
Fair Value, Inputs, Level 2 | Corporate Debt Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets | $ 70,574 | $ 32,916

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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